The balance of water and electrolytes is crucial for managing seriously ill patients. Measurement of sodium, potassium, urea creatinine, and bicarbonate levels provides important information about fluid, electrolyte, and renal function. Water makes up the majority of body composition, with 42 liters in a 70kg man, distributed between intracellular and extracellular compartments. The balance of water is maintained by input and output control, regulated by hormones like ADH and corticosteroids.